Ultrasonic imaging system of bonding zone
BACKGROUND OF THE INVENTION 
     The present invention relates to a system for imaging defects in a material of a plate- and layer-shaped structure by area-scanning an ultrasonic beam through a liquid medium and, more particularly, to an ultrasonic imaging system having an arithmetic operation control unit suitable for the high resolution imaging of a material of a distorted surface. 
     A conventional ultrasonic flaw detector system to three-dimensionally scan a test sample having a curved shape has been disclosed in JP-A-46-37318. This system discloses a basic method in which phases of transmission signals of a plurality of adjacent transmitting elements are so controlled that a focal point is formed in the sample and the image is displayed by the echoes from the focal point zone. However, no consideration is give to a method of controlling the focal point to an arbitrary depth under the surface of the sample. On the other hand, the principle that the focal point can be moved has also been known in JP-A-57-141549. However, this method has a drawback such that when the distance to the surface (incident point to a sample) changes, the focal point moves and this movement is not linear due to the difference of the sound velocities. Thus, this movement cannot be corrected by known means. 
     When a bonding zone of a layered structure of a test sample is extended along the imaging surface of the sample, in general, the distance from the imaging surface to the bonding zone to be imaged (hereinafter, referred to as an imaging object zone) is approximately known from a design value or the like. However, when the imaging surface is distorted or is a curved surface or when the imaging surface is inclined although it is flat surface, the distance in the liquid medium from the ultrasonic irradiating surface of the ultrasonic converter to the imaging surface (hereinafter, this distance is referred to as a medium distance) is not constant. The sound velocities in the liquid medium and in the sample differ. Therefore, the movement of the focal point is not linear. It is difficult to control the movement of the focal point so as to focus onto the imaging object zone. For instance, when the liquid medium is water and the sample is a silicon crystal plate, the underwater sound velocity v w  =1.5 mm/μsec and the sound velocity v sl  of the longitudinal wave in silicon is v sl  =8.4  mm/μsec and v sl  is 5.6 times as large as v w . Therefore, even if the distortion of the imaging surface is about 20 μm, an error of about 110 μm occurs in silicon. Thus, the error is magnified. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to eliminate the drawbacks of the conventional techniques and to provide an ultrasonic imaging system which can control a focal zone in a sample irrespective of a slight distortion of the surface shape. 
     According to the present invention, there is provided an ultrasonic imaging system of a bonding zone including ultrasonic transmitting/receiving means for focusing an ultrasonic beam and transmitting and receiving it. A pulse generating means is included for supplying high frequency pulses at a time set in the ultrasonic transmitting/receiving means. A receiving means is included for receiving and amplifying the ultrasonic signal received by the ultrasonic transmitting/receiving means. A switching means is also included for connecting the pulse generating means with the ultrasonic transmitting/receiving means in the transmitting mode and for connecting the ultrasonic transmitting/receiving means with the receiving means in the receiving mode. A liquid medium is interposed between the ultrasonic irradiating surface of the ultrasonic transmitting/receiving means and a test sample and an image of an imaging object zone in the sample is produced. This ultrasonic imaging system further includes input means for inputting parameters regarding sound velocities and dimensions of the test sample and liquid medium, a surface echo counter for extracting the echo from the sample surface from an output signal of the receiving means and for measuring the distance between the ultrasonic irradiating surface and the test sample from the transmission timing of the ultrasonic beam and the reception timing of the surface echo, and focal point arithmetic operation control means responsive to the distance measured by the surface echo counter and the output signal from the input means for outputting a signal to control the focal point of the ultrasonic beam. 
     That is, this system includes means for extracting the surface echo of the sample to be imaged and for measuring the time from the transmission of the ultrasonic beam to the reception of the surface echo and means for calculating and controlling the focal point of the ultrasonic beam in order to focus onto the imaging object zone in the sample from the measured time. The time at which the ultrasonic beam passes through the surface is detected. The conditions such as medium distance which is necessary to focus the beam onto the imaging object delay time of a transmission/reception signal of each vibrating element, and the like are arithmetically calculated based upon such principals as Snell&#39;s law or the like from the previously input parameters of the distance between the imaging object zone and the imaging surface, as well as the sound velocity in the sample, and the sound velocity in the liquid. On the basis of the arithmetic calculations, the position of the ultrasonic transmitting/receiving means, the transmission timing of each element, the phase of the reception signal, and the like are controlled. In this manner, the object of the invention is accomplished. 
     The surface echo counter calculates the time from the transmission of the ultrasonic pulse from the ultrasonic transmitting/receiving means to the reception of the surface echo. The delay time which should be given to the transmission/reception signal of each vibrating element can be obtained from an equation on the basis of the Snell&#39;s law such that the ultrasonic beam passes through a path in order to minimize the propagating time from the vibrating element through the sample surface to the focal point. This equation is generally a high-order equation which needs a convergence calculation to be performed in order to solve it. However, by preliminarily calculating a table of a sufficient accuracy from the parameters previously described and storing this table into a memory, and by driving the medium distance measured by the surface echo counter, the delay amount based on the distortion of the sample surface can also be determined. The phase of the transmission/reception signal can be controlled by the delay time obtained as mentioned above, so that a focal zone can be formed to an arbitrary depth in the sample. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an embodiment of the present invention; 
     FIG. 2 is a diagram for explaining an influence which is exerted by a change in medium distance on a delay time to be given to a transmission/reception signal of each element of an array converter; 
     FIG. 3 is a diagram for explaining an influence which is exerted on a medium distance gate opening/closing time; and 
     FIG. 4 is a block diagram for explaining a developed embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described hereinbelow with reference to FIGS. 1 to 3. The details of FIGS. 2 and 3 will be described later. Parameters which are input to input means 1 include: a sound velocity v 1  in a liquid medium; a sound velocity v 2  in a sample 2; a distance Z f  from the surface of the sample 2 to an imaging object zone; the number n of elements which are simultaneously driven among N vibrating elements of an array transducer 3; and an element pitch P. A delay correction amount arithmetic operating control section 4 calculates a delay time to be given to the transmission/reception signal of each of the vibrating elements which are simultaneously driven from those parameters which are output from the input means 1 and from the initial value of the medium distance Z w  shown in FIG. 3, thereby controlling a multichannel pulse generator (hereinafter, referred to a pulse generator) 5 as and a multichannel receiver (hereinafter, referred to as a receiver) 6. 
     The transmitting/receiving operation of an ultrasonic beam from the array transducer 3 is started from a transmission timing signal which is determined by a transmission timing signal generator 7. The vibrating elements are simultaneously driven and determined by an electronic scanning control section 8 and each channel of the pulse generator 5 is connected to a matrix switch 9. Each channel of the pulse generator 5 drives each of the simultaneously driven vibrating elements of the array transducer 3 according to the delay time and generates an ultrasonic beam 30 as shown in FIG. 3. The ultrasonic beam 30 progresses in the liquid medium while it gradually converges. Finally, this ultrasonic beam reaches an imaging surface 21 of the sample 2. In general, since the sound velocity in the sample differs from that in the liquid medium, the degree of convergence of the ultrasonic beam 30 also differs. A part of the ultrasonic beam is reflected by the imaging surface 21 and returned as a surface echo 31 to the array transducer 3 and the remaining ultrasonic beam is further propagated in the sample 2 and reaches the imaging object zone 22, for example a bonding zone, so that an echo 32 is produced. The echoes 31 and 32 are propagated along the same path and received by the array transducer 3 and converted into electric signals. Returning to FIG. 1, the matrix switch 9 connects each of the simultaneously driving vibrating elements with each channel of the receiver 6. The reception echoes are amplified while being subjected to phase correction according to the delay time by the receiver 6 and synthesized. In the output of the receiver 6, the signal which first appears after disappearance of the transmission pulse wave which is generated by the transmission timing signal generator 7 and subsequently was attenuated on the time axis of FIG. 3(b) or (c), corresponds to the surface echo 31. This echo is extracted by a surface echo counter 10. The medium distance Z w  is measured from the time difference between the transmission timing signal and the extracted surface echo 31. A correction amount of delay time to be added to the transmission/reception signal of each vibrating element is newly calculated from the measured medium distance Z w  and the parameters which have been previously input. This new correction amount is set as control data in the next transmission/reception cycle. By repeating these operations, the scanning of the ultrasonic beam is electronically controlled so as to converge the ultrasonic beam onto the imaging object zone. 
     The distance from the imaging surface 21 of the sample 2 to the imaging object zone is known as a design value. If Z w  is known, the time until the reception of the echo (hereinafter, referred to as an imaging echo) from the imaging object zone is calculated by a gate time arithmetic operation control section 11. The gate is opened or closed by a gate circuit 12 before and after the reception of this echo. The gate circuit 12 extracts the echo received for the period of time when the gate is open and outputs its amplitude to an image display section 13. In the image display section 13, the amplitude of the imaging echo which is received is written as a luminance to the coordinates (x&#39;, y&#39;) on the image display screen corresponding to the coordinates (x, y) of the center of the group of vibrators which are simultaneously driven in the array transducer 3, thereby forming and displaying an imaging picture. A digital display device 14 displays information regarding the imaging object zone. That is, depths f, a, and b from the surface which respectively correspond to the focal point, time to open the gate, and time to close the gate are displayed as numerals. 
     In FIG. 2, the central element in the group of n vibrators which are simultaneously driven is numbered 1. The other elements are sequentially numbered until n/2 for both sides. A vibrator pitch is set to P. Thus, the i-th (i=1, 2, . . . , n/2) vibrating element is located at the distance of x i  from the center of the vibrator group. 
     
         x.sub.i =i·P                                      (1) 
    
     Assuming that the focal point is set to the imaging object zone, it is sufficient to set the focal point F to a position which is just under the center of the vibrator group at the depth of Z f  from the imaging surface 21. The coordinates at which the wave which is propagated from the element i to the focal point F passes through the surface 21 are (x i0 , 0). Snell&#39;s law is satisfied between an incident angle θ 1  and an angle θ 2  of refraction= 
     
         v.sub.2 sinθ.sub.1 =v.sub.1 sinθ.sub.2         (2) 
    
     Further, 
     
         x.sub.i0 =Z.sub.f ·tanθ.sub.2               (3) 
    
     
         x.sub.i =Z.sub.f ·tanθ.sub.2 +Z.sub.w ·tanθ.sub.1                                (4) 
    
     By using the equations (1) to (4), a propagating time t i  of the ultrasonic beam from the element i to the focal point F is obtained by the following equation. ##EQU1## The delay correction amount among the elements is obtained as the relative value. It is obvious that the correction amount also changes due to a change in Z w . 
     In FIG. 3, the distance Z w  between the array transducer 3 and the surface 21 of the sample 2 changes to Z w  &#39; in the distorted portion on the surface. On the other hand, it is assumed that the distance Z f  between the surface 21 of the sample and the bonding zone 22 is hardly influenced by the presence or absence of the distortion. As shown in FIG. 3(a), the ultrasonic pulses 30 emitted from the array transducer 3 are partially reflected by the surface 21 and returned as the surface echo 31 to the array transducer 3. 
     The remaining pulses 30 are propagated in the sample 2 and are further partially reflected by a bonding zone 22 and received as the imaging echo 32 by the array transducer 3. When the reception echo is monitored on a time base, as shown in FIG. 3(b), the surface echo 31 appears at the position of t 0  and the imaging echo 32 appears at the position of t f . When the medium distance Z w  changes to Z w  &#39; due to the distortion of the surface 21, as shown in FIG. 3(c), the echoes 31 and 32 move as shown at the positions t 0  &#39; and t f  &#39;, respectively. The opening/closing times of the gate also need to be corrected in correspondence to the delay times. In this case, by carrying out this correction, the imaging object zone can be displayed in the image display section 13. 
     According to the embodiment, Z w  is always monitored by the surface echo counter and the delay correcting time and the gate opening/closing times are respectively arithmetically operated and controlled. Thus, the focal point can be always set to an arbitrary position in the sample and the imaging picture can be formed by the echo from the focal zone. 
     FIG. 4 shows another embodiment of the invention. The mechanism 15 to hold the array transducer 3 further has: a control circuit arithmetic operating section which functions so as to keep constant the distance between the sample surface 21 and the array transducer 3 in accordance with the value of Z w  which is output from the surface echo counter 10; and a Z axis drive control unit which is driven by an ultrasonic motor or the like to move the array transducer 3 on the basis of the result of the arithmetic operation. 
     According to this embodiment, it is possible to accurately correct for a fine change in Z w . 
     According to the invention, even in the case of a distorted or inclined sample surface or a curved surface where the control of the focal point position was hitherto difficult, the focal point can be set to an arbitrary surface in the sample at an accuracy higher than with the conventional imaging system and this surface can be imaged at a high resolution. Thus, there is an advantage such that a finer imaging picture is obtained.